1. Field of the Invention
This invention relates to a light beam position control device for a beam deflector.
2. Background of the Invention
An example of a conventional light beam position control device for a beam deflector will be described with reference to FIG. 1,
As shown in FIG. 1, light beams 1 and 2 emitted from light sources 11 and 12 are collimated by condenser lenses 13 and 14, and respectively reflected by movable reflectors 5 and 6. The light beams 1 and 2 thus collimated are applied to a beam splitter 15 so as to be split into beams advancing towards a beam deflector 16 and beams advancing towards photo detectors 3 and 4. The beam deflector 16 is a rotating polygonal mirror. The beams advancing towards the beam deflector 16, after being reflected, are focused on a drum 18 by an f-.theta. lens 17 or the like, thus defining two scanning lines as the beam deflector 16 rotates. On the other hand, the beams advancing towards the photo detectors 3 and 4 are focused on the photo detectors 3 and 4 by a focusing lens 19. The photo detectors 3 and 4 form a position detecting sensor for maintaining the relative position of the two light sources 11 and 12 constant. The surface of the drum 18 and the photo detectors 3 and 4 are in the focusing planes of the lens systems 17 and 19, respectively. The beam diameters and the beam distance on the drum 18 are equal to those on the photo detectors 3 and 4, respectively. In order to position the beams 1 and 2 adjacent to each other on the drum 18, the beams are positioned adjacent to each other on the photo detectors 3 and 4.
When the beams 1 and are displaced on the photo detectors 3 and 4, the differential output of each of the photo detectors 3 and 4 is applied to a servo circuit 23, which outputs a corresponding servo signal. The servo signal operates actuators 7 and 8 moving the movable reflectors 5 and 6 so that the position of the beams 1 and 2 on the photo detectors 3 and 4 are corrected.
A conventional method of positioning the beams on the photo detectors is as shown in FIG. 2. When the power switch has been turned off, the positions of the beams 1 and 2 are not constant. When the power switch is turned on, the servo circuit 23 operates to suspend the application of the output signals of the photo detectors 3 and 4 to the movable reflectors 5 and 6, and to move the movable reflectors 5 and 6 so that the first beam 1 is applied to the first photo detector 3 which should receive the beam 1 and the second beam 2 is applied to the second photo detector 4 which should receive the second beam 2. Upon detection of the beams by the photo detectors 3 and 4, the servo circuit 23 is so operated that the movable reflectors 5 and 6 are moved according to the output signals of the photo detectors 3 and 4. This operation is, however, effected even when the first beam 1 is applied to the second photo detector 4 while the second beam 2 is applied to the first photo detector 3. This erroneous operation may be eliminated by a method in which, before the beams 1 and 2 are positioned on the photo detectors 3 and 4, the beam 1 is placed above the first photo detector 3 while the second beam 2 is placed below the second photo detector 4. However, the method makes the beams positioning operation correspondingly intricate.